Diablo III dev blog demystifies the Mystic
Through Enchantment, the Mystic is capable of rerolling a single property on a piece of equipment, making her the go-to artisan for those items that are almost-but-not-quite perfect. While you can choose only a single property to reroll on each item, that property can be rerolled as many times as you can afford. Don't worry about buyer's remorse, though; if the newly rolled property is worse than the old one, the Mystic can reverse the process. The Mystic is also the number one stylist in Sanctuary thanks to her transmogrification ability which, as World of Warcraft players know, allows players to replace an item's visual appearance with that of a different item. To get all the arcane details on Myriam the Mystic, you can check out the full post on Diablo III's official site.